Grating measurement system is widely used in various electromechanical devices as a typical displacement sensor. The measurement principle of the grating measurement system is mainly based on Moiré principle and principle of diffraction and interference. The grating measurement system based on Moiré principle is preferred for displacement measurement of various electromechanical devices as a fully developed displacement sensor due to its numerous advantages such as high resolution, high precision, lower cost and easiness to install and adapt.
In semiconductor manufacturing equipment, a lithography machine is the key device in semiconductor chip manufacture. An ultra-precision workbench is the core subsystem of the lithography machine, and is used for carrying the mask plates and silicon wafer to perform high speed ultra-precision stepping scanning movements. The ultra-precision workbench is the most typical system in the ultra-precision movement system due to its movement characteristics, such as high speed, high acceleration, large-stroke, ultra-precision, multiple degrees-of-freedom. In order to achieve the above movements, a dual-frequency laser interferometer measurement system is typically utilized for measuring multiple-degrees-of-freedom displacement of the ultra-precision workbench in the ultra-precision workbench. However, as the movement indexes for measurement preciseness, measurement distance and measurement speed increase continually, the dual-frequency laser light interferometer is difficult to meet the measurement requirement due to a series of problems, such as environmental sensitivity, difficulties in improving measurement speed, space occupation, high price, difficulties in designing and manufacturing a workpiece table as a measuring target.
In view of the above problems, a series of researches, concentrating mainly on grating measurement systems based on diffraction and interference principle, are performed by a lot of large corporations and research institutes in the field of ultra-precision measurement all around the world, and research achievements are disclosed in numerous patents and papers. U.S. patents owned by ASML, Netherland, U.S. Pat. No. 7,102,729B2 (publication date: Aug. 4, 2005), U.S. Pat. No. 7,483,120B2 (publication date: Nov. 15, 2007), U.S. Pat. No. 7,940,392B2 (publication date: Dec. 24, 2009), US2010/0321665A1 (publication date: Dec. 23, 2010) disclosed a plane grating measurement system and arrangement scheme for lithography machine ultra-precision workbench, which mainly utilizes a one or two dimensional plane grating to measure the long-stroke horizontal displacement of the workpiece stage with the cooperation of a reading head, and utilizes height sensors, such as eddy current or interferometers, to measure displacement in height direction, however, the measurement precision of the workpiece stage was limited by the application of various sensors. ZYGO, American company, discloses a grating measurement system applied to a ultra-precision workbench for lithography machine, which also utilizes a one or two dimensional plane grating to measure displacement of the workbench with the cooperation of a specific reading head, and is capable of performing horizontal and vertical displacement measurement at the same time, in US patent publication number US2011/0255096A1 (publication date: Oct. 20, 2011). CANON, Japanese company, discloses a heterodyne interferometer, which utilizes a grating as an target lens, and could only perform one dimensional measurement, in US patent publication number US2011/0096334A1 (publication date: Apr. 28, 2011). GAOWEI, Japanese scholar, proposes a single-frequency two-dimensional grating measurement system utilizing diffraction and interference principle, which is capable of performing horizontal and vertical displacement measurement at the same time, and the preciseness of which was difficult to be guaranteed due to utilization of single frequency laser and measurement signal being susceptible to disturbance, in “Design and construction of a two-degree-of-freedom linear encoder for nanometric measurement of stage position and straightness. Precision Engineering 34 (2010) 145-155”.
In view of the limitations on the above technical solution, a heterodyne grating interferometer measurement system utilizing optical beat frequency theory capable of achieving sub-nanometer and even higher resolution and preciseness and measuring long-stroke horizontal displacement and vertical displacement at the same time is required. By using the measurement system as the displacement measurement device for a ultra-precision workbench, the defects of laser interferometer measurement systems in ultra-precision workbench can be diminished effectively, thereby improving the performance of the ultra-precision workbench of the lithography machine.